Method and apparatus for preventing cracking in a liquid cooling system

ABSTRACT

An apparatus for preventing cracking of a liquid system includes an enclosure and one or more compressible objects immersed in the enclosure. According to the present invention, the enclosure is configured to cause a fluid to begin to freeze at a location in the enclosure, and for freezing to advance towards the one or more compressible objects.

RELATED APPLICATION

This patent application is a continuation-in-part of the co-pending U.S. patent application Ser. No. 11/049,202, filed on Feb. 1, 2005, and titled “METHOD AND APPARATUS FOR CONTROLLING FREEZING NUCLEATION AND PROPAGATION,” which claims priority under 35 U.S.C. § 119(e) of the U.S. provisional patent application Ser. No. 60/577,262, filed on Jun. 4, 2004, and titled “MULTIPLE COOLING TECHNIQUES,” both of which are hereby incorporated by reference. Also, this patent application is a continuation-in-part of the co-pending U.S. patent application Ser. No. 10/643,641, filed on Aug. 18, 2003, and titled “REMEDIES TO PREVENT CRACKING IN A LIQUID SYSTEM,” which claims priority under 35 U.S.C. § 119(e) of the U.S. provisional patent application Ser. No. 60/444,269, filed on Jan. 31, 2003, and titled “REMEDIES FOR FREEZING IN CLOSED-LOOP LIQUID COOLING FOR ELECTRONIC DEVICES,” both of which are hereby incorporated by reference.

FIELD OF THE INVENTION

The present invention relates to an apparatus and method of preventing cracking of a liquid system, such as may be useful for transferring heat from electronic devices and components thereof. In particular, the invention protects against expansion of fluid during freezing by including a variety of means and objects to protect against expansion of water-based solutions when frozen and by initiating the expansion of frozen fluid in the direction of zones having generally decreasing surface area to volume ratios.

BACKGROUND OF THE INVENTION

When water or many other liquid mixtures are cooled below their freezing points, the material changes from a liquid state to a solid state, and undergoes a significant expansion in volume. Water that has frozen in pipes or other confined spaces does more than simply clog the pipes and block flow. When freezing occurs in a confined space like a steel pipe, the ice will expand and exert extreme pressure which is often enough to crack the pipe and cause serious damage. This phenomenon is a common failure mode in hot-water heating systems and automotive cooling systems.

Ice forming in a pipe does not always cause cracking where ice blockage occurs. Rather, following a complete ice blockage in a pipe, continued freezing and expansion inside the pipe can cause water pressure to increase downstream. The increase in water pressure leads to pipe failure and/or cracking. Upstream from the ice blockage the water can retreat back towards its inlet source, and there is little pressure buildup to cause cracking.

Liquid cooling systems for electronic devices are occasionally subjected to sub-freezing environments during shipping, storage, or in use. Since these systems are going to be frozen on occasion, they must be designed to tolerate the expansion of water when frozen. Additives, such as antifreeze, are potentially poisonous and flammable and can damage mechanical components, sensitive sensors, and electronics, which is why pure or substantially pure water is typically the coolant of choice.

What is needed is an apparatus for and method of preventing cracking in a liquid cooling system that can tolerate a predetermined level of freezing and expansion inside confined spaces without damaging electronic components or affecting system performance.

SUMMARY OF THE INVENTION

The present invention protects components and pipes of a liquid cooling system from cracking related to an expansion of volume due to freezing of the fluid within the system. In particular, one aspect of the present invention provides an apparatus for and method of controlling freezing nucleation and propagation in a liquid system having one or more components coupled and characterized by a plurality of surface area to volume ratios so that when freezing occurs, the fluid expands from an initial zone having a highest surface area to volume ratio in the direction of one or more zones having progressively decreasing surface area to volume ratios. Thus, one aspect of the present invention manages and designs surface area to volume ratios of one or more components as well as regions within the components, including heat exchangers, inlet and outlet ports and tubular members, so that when freezing occurs, the volume expands in the direction that can accept the expanded volume. Additionally, another aspect of the present invention provides an apparatus and method for forming a liquid cooling system that utilizes size and volume reducing means, air pockets, compressible objects, and flexible objects to protect against expansion of water-based solutions when frozen. In such a system, pipes, pumps, and heat exchangers are designed to prevent cracking of their enclosures and chambers.

In one aspect, an apparatus for preventing cracking of a liquid system is disclosed. The apparatus includes an enclosure and a compressible object. The enclosure is configured to have multiple zones of different freeze susceptibilities and to cause freezing to begin in a high freeze susceptibility zone and for a freeze front to advance from the high freeze susceptibility zone toward a low freeze susceptibility zone through one or more zones of progressively decreasing freeze susceptibility. The compressible object is immersed in a zone of lower freeze susceptibility than the high freeze susceptibility zone.

In another aspect, another apparatus for preventing cracking of a liquid system is disclosed. The apparatus includes an enclosure and a pressure relief area. The enclosure is configured to have multiple zones of different surface area to volume ratios and to cause freezing to begin in a high surface area to volume ratio zone and for a freeze front to advance from the high surface area to volume ratio zone toward a low surface area to volume ratio zone. The pressure relief area is positioned within the enclosure and in a zone other than the high surface area to volume ratio zone. The pressure relief area can be a compressible object.

In yet another aspect, a freeze-tolerant heat exchanger is disclosed. The heat exchanger includes a micro-structured heat exchange region having a first freeze susceptibility, a manifold region configured to have a second freeze susceptibility so that fluid within the manifold region freezes later than fluid within the micro-structured heat exchange region, and a fluid input region including a compressible object and configured to have a third freeze susceptibility so that fluid within the fluid input region freezes later than fluid within the manifold region, wherein the heat exchanger is configured so that a freeze front advances from the micro-structured heat exchange region towards the compressible object. The micro-structured region can include one or more of microchannels, microporous foam, and pseudo-foam.

In another aspect, a method of preventing cracking of a liquid system is disclosed. The system includes a pump and a heat exchanger. The method includes configuring the system to have multiple zones of different surface area to volume ratios and to cause freezing to begin in a high surface area to volume ratio zone and to advance towards a low surface area to volume ratio zone. The method also includes providing an enclosure fluidly coupled to the system at a zone other than the high surface area to volume ratio zone, and placing a compressible object in the enclosure.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 illustrates a schematic diagram of a conventional closed-loop cooling system, which includes a pump and a heat exchanger.

FIG. 2 illustrates one embodiment of a heat exchanger divided into logical zones characterized by surface area to volume ratios, in accordance with the present invention.

FIG. 3 illustrates a schematic diagram of a housing having an inlet chamber and an outlet chamber.

FIG. 4 illustrates a schematic diagram of a housing having inlet and outlet chambers reduced in size and volume in accordance with the present invention.

FIG. 5 illustrates a schematic diagram of an air pocket disposed in an inlet chamber and an outlet chamber of a housing in accordance with the present invention.

FIG. 6 illustrates a schematic diagram of a compressible object disposed in an inlet chamber and an outlet chamber of a housing in accordance with the present invention.

FIG. 7A illustrates a schematic diagram of a housing having inlet and outlet chambers and a plurality of spaced apart flexible objects coupled to the chambers.

FIG. 7B illustrates a schematic diagram of a housing having inlet and outlet chambers and a plurality of spaced flexible objects coupled to the chambers, the flexible objects being displaced during fluid expansion to prevent cracking.

FIG. 8A illustrates a schematic diagram of compressible objects coupled to inlet and outlet ports within a heat exchanger.

FIG. 8B illustrates a schematic diagram of compressible objects disposed along a bottom surface of a heat exchanger within adjacent microchannels.

FIG. 9A illustrates a schematic diagram of compressible objects coupled to walls of fluid filled tubing within a heat rejector.

FIG. 9B illustrates a schematic diagram of compressible objects disposed along a length of fluid filled tubing within a heat rejector.

FIG. 10 illustrates a schematic diagram of compressible objects disposed within fluid filled channels of a plate within a heat rejector.

FIG. 11 illustrates a schematic diagram of compressible objects disposed in fluid segments of a cooling loop.

FIG. 12 illustrates a schematic diagram of a housing having an inlet chamber and an outlet chamber and a plurality of spaced apart flexible objects coupled to the chambers.

FIG. 13 illustrates a schematic diagram of a housing having inlet and outlet chambers and a plurality of spaced apart flexible objects coupled to the chambers, the flexible objects being displaced during fluid expansion to prevent cracking.

FIG. 14 illustrates a flow chart illustrating steps of a preferred method of one embodiment of the present invention.

FIG. 15 illustrates a schematic diagram of a housing having inlet and outlet chambers having a relatively narrowed central portion and substantially identical expanded end portions.

DETAILED DESCRIPTION OF THE INVENTION

Reference will now be made in detail to the preferred and alternative embodiments of the invention, examples of which are illustrated in the accompanying drawings. While the invention will be described in conjunction with the preferred embodiments, it will be understood that they are not intended to limit the invention to these embodiments. On the contrary, the invention is intended to cover alternatives, modifications and equivalents, which may be included within the spirit and scope of the invention as defined by the appended claims. Furthermore, in the following detailed description of the present invention, numerous specific details are set forth in order to provide a thorough understanding of the present invention. However, it should be noted that the present invention may be practiced without these specific details. In other instances, well known methods, procedures and components have not been described in detail as not to unnecessarily obscure aspects of the present invention.

FIG. 1 shows a schematic diagram of a closed-loop cooling system 100, which includes heat exchanger 20 attached to a heat producing device 55 (shown as an integrated circuit attached to a circuit board, but which could also be a circuit board or other heat producing device), a pump 30 for circulating fluid, a heat rejector 40, which can include a plurality of fins 46 for further assisting in conducting heat away from the system 100, and a controller 50 for a pump input voltage based on a temperature measured at the heat exchanger 20. Fluid flows from an inlet 32, is pulled through a porous structure (not shown) with the pump 30, and exits through the outlet 34. While the preferred embodiment uses an electroosmotic pump, it will be understood that the present invention can be implemented in a system using other types of pumps.

Still referring to FIG. 1, the fluid travels through the heat exchanger 20 and the heat rejector 40 through tubing lengths 114 and 110 before being recycled back to the inlet 32 of the pump 30 via another tubing 112. The controller 50 is understood to be an electronic circuit that takes input signals from temperature sensors in the heat exchanger 20, or from temperature sensors in the device 55 being cooled, which signals are transmitted along signal lines 120. The controller 50, based upon the input signals regulates flow through the pump 30 by applying signals to a power supply (not shown) associated with the pump 30 along signal lines 122 to achieve the desired thermal performance. While this embodiment specifies a flow direction, it will be understood that the present invention can be implemented with the reverse flow direction.

As fluid temperature drops below freezing, ice forms into a blockage. The rate at which ice forms depends on the rate at which the fluid cools, which depends at least in part on a surface area to volume ratio. Continued growth of ice in areas of the system 100 can lead to excessive fluid pressure. The resulting pressure can rupture or damage individual elements, such as the lengths 110, 112, 114 of tubing, channels in the heat exchangers 20 and 40, and/or chambers inside the pump 30. As will be explained and understood in further detail below, the individual elements must be designed in a way that tolerates expansion of the fluid or water when frozen.

FIG. 2 illustrates one embodiment of a heat exchanger 200 divided into zones 1, 2, 3A and 3B and characterized by surface area to volume ratios. The heat exchanger 200 is coupled to tubular members 210 and 260 disposed in zone 4A and 4B, respectively, and also characterized by surface area to volume ratios. In this embodiment, zone 1 is the initial zone and the tubular members represent a final zone or zones. Zone 1 is preferably one or more microchannels (not shown) or a porous structure (not shown), such as microporous foam or pseudo-foam. Alternatively, zone 1 can be one or more micropins (not shown). Surface areas are calculated for each zone, preferably based directly on model geometry. A zone can be constructed of one or more structures, such as copper foam, to have a desired surface area to volume ratio throughout the heat exchanger 200. Volumes are calculated for each zone, preferably based directly on model geometry. The surface area to volume ratio of each zone is calculated by dividing the surface area of each zone by the volume of each zone. The resulting surface area to volume ratio values of adjacent zones are compared. Freeze progression is deemed favorable when the surface area to volume ratio of the heat exchanger 200 progressively decreases outward from zone 1 to the tubular members at the onset of freezing. In particular, the surface area to volume ratio of zone 1 is relatively high and the surface area to volume ratios of the tubular members (zones 4A, 4B) are relatively low.

During freezing, the fluid expands from a zone having the highest surface area to volume ratio in the direction of one or more zones having progressively decreasing surface area to volume ratios. It will be appreciated that the heat exchanger 200, including the tubular members 210 and 260, can include many zones each with a different surface area to volume ratio. The zone surface area to volume ratio of adjacent zones progressively decreases from the heat exchanger 200 in the direction of the tubular members 210 and 260; the zone surface area to volume ratio decreases in the following order of zones: 1>2>3B>4B and 1>2>3A>4A. In this embodiment, the tubular members 210 and 260 are designed to accommodate the necessary volume expansion.

The tubular members 210 and 260 preferably include compliant materials to accommodate an expanded volume equivalent to at least the cumulative change in volume of the freezing liquid in the system. Preferably, the tubular members 210 and 260 have elasticity sufficient to expand outwardly to accommodate the volume expansion caused by the freezing of the fluid. Alternatively, the one or more compressible objects (not shown) can be coupled to the tubular member 210 and 260 wherein pressure exerted on the compressible object by the freezing fluid increases a volume of the tubular members 210 and 260. Preferably, the compressible objects (not shown) are confined within the tubular member and can be made of closed cell sponge, closed cell foam, air-filled bubbles, sealed tubes, balloons and/or encapsulated in a hermetically sealed package. The package can be made of metallic material, metallized plastic sheet material, or plastic material. The plastic materials can be selected from teflon, mylar, nylon, a laminate of CTFE and PE, PET, PVC, PEN or any other suitable package. Other types of compressible objects can be used. The sponge and foam can be hydrophobic.

In another embodiment, at least one air pocket (not shown) can be disposed in the tubular members 210 and 260 wherein the air pocket (not shown) accommodates the expansion by the freezing fluid. Alternatively, at least one flexible object (not shown) is coupled to the tubular members 210 and 260 wherein pressure exerted on the flexible object (not shown) by the freezing fluid increases a volume of the tubular members 210 and 260. The flexible object (not shown) is preferably secured within the tubular member and made of one of the following: rubber, plastic, and foam. It will be appreciated that additional compliant materials may also be employed to withstand the expansion of freezing fluid.

In one embodiment, shown in FIG. 3, an apparatus or pump 60 includes a housing 68 having an inlet chamber 62 and an outlet chamber 64. A pumping mechanism or structure 69 separates the inlet and outlet chambers 62 and 64 from a bottom surface of the housing 68 to an upper surface of the housing 68. The pumping structure 69 channels liquid from a pump inlet 61 to a pump outlet 66. The chambers 62 and 64 are filled with fluid. Preferably, the liquid used in the pump 60 is water. It is contemplated that any other suitable liquid is contemplated in accordance with the present invention.

Still referring to FIG. 3, the pump 60 can be designed so that there are no large pockets of water in any of the chambers 62 and 64. Since water expands as it freezes, ice takes up more room than liquid. When freezing occurs in confined spaces, such as chambers 62 and 64, displacement caused by the expansion of fluids is proportional to an amount of fluid volume in the chambers 62 and 64. Minimizing the size and volume occupied by the chambers 62 and 64 reduces the displacement, and thereby minimizes the amount of liquid displaced within the chambers 62 and 64 by freezing.

As shown in FIG. 4, the volume of inlet and outlet chambers 72 and 74 is substantially reduced compared to the chambers 62 and 64 in FIG. 3. As such, the amount of water present in the pump 70 is greatly reduced. Detailed mechanical analysis of the chambers 72 and 74 is required, but the chambers 72 and 74 can be designed to withstand force exerted by frozen water. The inlet and outlet chambers 72 and 74 can be capable of contracting and expanding between a minimum size and volume condition and a maximum size and volume condition. It should be understood that the tubing lengths 110, 112, and 114 in FIG. 1 can be reduced in size and volume to reduce displacement caused by fluid expansion in areas of the system 100 (FIG. 1).

In another embodiment, as shown in FIG. 5, an apparatus or pump 80 includes a housing 88 having an inlet chamber 82 and an outlet chamber 84. A pumping structure 89 separates the inlet and outlet chambers 82 and 84 from a bottom surface of the housing 88 to an upper surface of the housing 88. The pumping structure 89 channels liquid from a pump inlet 81 to a pump outlet 86. The chambers 82 and 84 are filled with fluid to a large extent. Preferably, the liquid used in the pump 80 is water. It is contemplated that any other suitable liquid is contemplated in accordance with the present invention.

Still referring to FIG. 5, air pockets 85 and 87 are disposed in the inlet and outlet chambers 82 and 84. The air pockets 85 and 87 are preferably positioned farthest away from a location where fluid begins to freeze in the chambers 82 and 84. Expansion of the ice upon freezing in the chambers 82 and 84 will take up some space occupied by the air pockets 85 and 87, and cause a slight increase of pressure in the chambers 82 and 84. However, air is compressible enough that it can be significantly compressed with relatively small forces, such that the expansion of the ice is easily accommodated. Preferably, the air pockets 85 and 87 have a volume proportional to an amount of fluid in the chambers 82 and 84. The air pockets 85 and 87 can preferably accommodate a predetermined level of fluid expansion between five to twenty five percent.

As mentioned before, ice forming in a confined space does not typically cause a break where initial ice blockage occurs. Rather, following a complete ice blockage in a confined space, continued freezing and expansion inside the confined space cause fluid pressure to increase downstream. The fluid pressure will reach a maximum at a last location to freeze in a hermetically sealed system. The pressure can be very large, unless there is a trapped air pocket in that region. Thermal design of the chambers 82 and 84 can be altered to select a location where the fluid begins to freeze, and to arrange for freezing to start from one location and advance continuously towards an air pocket at another location. For example, if there is an air pocket at the top surface of a chamber, the fluid should be nucleated at the bottom surface of the chamber. As the fluid begins to freeze at the bottom surface of the chamber, ice expansion displaces water and compresses the air pocket. Since air is easily compressible, the chamber can freeze completely without generating large forces at any location in the chamber.

To arrange a location of initial freezing in the chamber, it may be necessary to provide a thermal path from the location of initial freezing to its surroundings. As the fluid or chamber is cooled from above a freezing point, the thermal path serves to efficiently reject thermal energy stored in the location. For example, an optional metallic insert 288 is mounted from the location of initial freezing in the chamber to the top surface of the chamber would serve. Preferably, the metallic insert 288 is formed of a material that will not contaminate the fluid, such as copper. Alternatively, locally increasing the surface to volume ratio of the chamber or reducing package insulation in the chamber could also work as a replacement for the metallic insert 288. A critical factor is use of any material or structure that assists a particular location become cold fastest, and so that progression of freezing is continuous from that location to the air pockets 85 and 87 of FIG. 5.

In some cases, it may be difficult to control the positioning and location of the air pockets 85 and 87 in the chambers 82 and 84. Further, it may be difficult to dispose an air pocket in each chamber of the system 100 (FIG. 1). In a further embodiment, as shown in FIG. 6, one or more compressible objects 95 and 97 are immersed in pump 90. The pump 90 includes a housing 98 having an inlet chamber 92 and an outlet chamber 94. A pumping structure 99 separates the inlet and outlet chambers 92 and 94 from a bottom surface of the housing 98 to an upper surface of the housing 98. The pumping structure 99 channels liquid from a pump inlet 91 to a pump outlet 96. The chambers 92 and 94 are filled with fluid to a large extent. Preferably, the liquid used in the pump 90 is water. It is contemplated that any other suitable liquid is contemplated in accordance with the present invention.

Still referring to FIG. 6, the one or more compressible objects 95 and 97 are immersed and coupled to inlet and outlet chambers 92 and 94. The objects 95 and 97 can be a closed cell hydrophobic foam or sponge. Preferably, the objects 95 and 97 accommodate a predetermined level of fluid expansion between five to twenty-five percent. To accommodate the fluid expansion, the objects 95 and 97 can preferably have a size and volume proportional to an amount of fluid in the chambers 92 and 94.

The objects 95 and 97 can be comprised of a compressible material, such as an open-cell or closed-cell foam, rubber, sponge, air-filled bubbles, elastomer, or any related material, and a protective layer covering all surfaces of the compressible material. A purpose of having the protective layer is to prevent contact between the compressible material and a surrounding fluid. The protective layer can be formed by many means, including wrapping and sealing, dip-coating, spray-coating, or other similar means. The protective layer can be a vacuum laminated cover, such as a spray-on layer, a deposited layer, or a layer formed by reacting or heating surfaces of the compressible material. In addition, it is possible to form a protective layer on the surface of the compressible material by thermally fusing, melting, or chemically modifying the surface. The protective layer can be flexible enough so that a volume of the compressible material can be reduced by pressure. In order to achieve this degree of flexibility, the protective layer can be much thinner than the compressible material. Further, the protective layer can be formed from a material that is not chemically attacked by the fluid used in the cooling system, or degraded by temperature cycles above and below freezing. The protective layer can be hermetically sealed so that gas cannot enter or leave the volume within the protective layer. The protective layer can be formed from a variety of materials, including teflon, mylar, polyethylene, nylon, PET, PVC, PEN or any other suitable plastic, and can additionally include metal films on interior or exterior surfaces to improve hermeticity. In addition, the protective layer can be a metallized plastic sheet material, as used in potato chip packaging, and can serve as an impervious layer, blocking all gas and liquid diffusion. Furthermore, in cases where occasional bubbles are moving through the cooling system, as when an electroosmotic pump is generating hydrogen and oxygen gas bubbles, the protective layer can be hydrophilic to help reduce the possibility that the bubbles will attach to the surfaces.

In a further embodiment, as shown in FIG. 7A, an apparatus or pump 103 includes a housing 108 having an inlet chamber 102 and an outlet chamber 104. A pumping structure 109 separates the inlet and outlet chambers 102 and 104 from a bottom surface of the housing 108 to an upper surface of the housing 108. The pumping structure 109 channels liquid from a pump inlet 101 to a pump outlet 106. The chambers 102 and 104 are filled with fluid to a large extent. Preferably, the liquid used in the pump 103 is water. It is contemplated that any other suitable liquid is contemplated in accordance with the present invention.

Still referring to FIG. 7A, a plurality of spaced apart flexible objects 105 and 107 are coupled to the inlet and outlet chambers 102 and 104. In this embodiment, the flexible objects 105 and 107 are preferably constructed from a flexible material, such as rubber or plastic. The flexible material is preferably designed and arranged such that it can be partially displaced, such as shown in FIG. 7B, to accommodate expansion of ice without cracking itself or other rigid elements of the inlet and outlet chambers 102 and 104. Preferably, the flexible objects 105 and 107 accommodate a predetermined level of fluid expansion between five to twenty five percent. The flexible objects can be spaced apart from one another a predetermined distance. Preferably, the flexible objects 105 and 107 are capable of contracting and expanding between a minimum volume condition and a maximum volume condition.

FIG. 8A illustrates a schematic diagram of compressible objects 132 and 134 coupled to inlet and outlet ports 131 and 135 within a heat exchanger 130. Fluid generally flows from one or more inlet ports 131 and flows along a bottom surface 137 in microchannels 138 of any configuration and exits through one or more outlet ports 135, as shown by arrows. The compressible objects 132 and 134 are preferably designed and arranged such that they can be partially displaced to accommodate expansion of ice without cracking themselves or other rigid elements of the inlet and outlet ports 131 and 135 in FIG. 8A.

FIG. 8B illustrates a schematic diagram of compressible objects 145 disposed along a bottom surface 147 of a heat exchanger 140 within microchannels 148. As shown in FIG. 8B, the compressible objects 145 can be arranged within the microchannels 148 such that the compressible objects 145 form part of a seal from a top surface 149 to the bottom surface 147. In both FIGS. 8A and 8B, compressible objects act as freeze protection within a heat exchanger. The positioning of the compressible objects 145 is intended to minimize flow resistance, and to avoid degrading heat transfer from the bottom surface 147 to the fluid. Placement of the compressible objects 145 on sides of the microchannels is also possible, although less advantageous than the positioning as shown in FIG. 8B. Positioning on the bottom surface 148 would severely degrade performance of the heat exchanger 140 because of a high thermal resistance of the compressible objects 145.

FIG. 9A illustrates a schematic diagram of compressible objects 152 and 154 coupled to walls 151 and 155 of fluid filled tubing 150 within a heat rejector. The tubing 150 can be substantially longer than other portions of the system, for example centimeters in length in certain parts of the system 100 (FIG. 1), and as much as a meters in length in other parts. Placement of a length of the compressible objects 152 and 154 to the walls 151 and 155 of the tubing 150 will act as freeze protection within a heat rejector. Alternatively, as shown in FIG. 9B, compressible element 165, such as compressible foam structures, can be threaded along a length of the tubing 160. The compressible element 165 can float freely within the tubing 160. Because the compressible element 165 is thinner than the tubing 160, it can simply be threaded without concern for forming a blockage in the tubing 160. A length of the compressible elements 165 will vary according to the lengths of the tubing 160.

FIG. 10 illustrates a schematic diagram of various possible configurations for compressible objects 171, 173, 175 and 177 disposed within fluid filled channels 170 of a plate 180 within a heat rejector. As shown in FIG. 10, fluid can be routed through the channels 170 disposed within the plate 180 that allows fluid flow between a fluid inlet 172 and a fluid outlet 174. A heat rejector can include fins 190 mounted to and in thermal contact with the plate 180. The compressible objects 171, 173, 175 and 177 disposed within the channels 170 provide freeze protection, thereby improving performance of the entire system.

In addition to the use of size and volume reducing means, air pockets, compressible objects, and flexible objects discussed above, other techniques can be used to prevent cracking in a liquid cooling system, as would be recognized by one of ordinary skill in the art. For example, as shown in FIG. 11, compressible elements 182 can partly fill all fluid segments of a cooling loop. In all these cases, it will be appreciated by one of ordinary skill that routine mechanical design analysis is useful to compute stress throughout the cooling system including but not limited to the chambers, lengths of tubing, and other enclosures that contain either the air pockets or compressible objects to design a system for which stresses do not accumulate in any location in sizes large enough to cause the enclosures to fail. In a closed-loop cooling system for an electronic device, relatively large reservoirs of fluid are likely to be in the chambers of the pump or the tubing in a heat exchanger. System design should strive to minimize these volumes of fluid, thereby reducing the volume of the compressible material used. Failing that, or if large volumes of fluid are needed to guarantee sufficient fluid over extended use, the embodiments described above can reduce forces generated during freezing to manageable levels.

In another embodiment, shown in FIG. 12, an apparatus or pump 200 includes a housing 208 having an inlet chamber 202 and an outlet chamber 204. A pumping structure 209 separates the inlet and outlet chambers 202 and 204 from a bottom surface of the housing 208 to an upper surface of the housing 208. The pumping structure 209 channels liquid from a pump inlet 201 to a pump outlet 206. The chambers 202 and 204 are filled with fluid. Preferably, the liquid used in the pump 200 is water. It is contemplated that any other suitable liquid is contemplated in accordance with the present invention.

Still referring to FIG. 12, the housing 208 can be designed to withstand expansion of the fluid when freezing occurs. A plurality of flexible objects 207 are coupled to at least one wall of the housing 208. The housing 208 consists of rigid plates and support the chambers 202 and 204. The plates make up a plurality of sides of the chambers 202 and 204 and are joined by the flexible objects 207. The flexible objects 207 can be fastened to the plates. The flexible objects 207 can be formed on any or each of the plurality of sides of the chambers 202 and 204, which includes corner edges, and allow the plates to be displaced outward when acted upon by force, as shown in FIG. 13. The flexible objects can be elastomer hinges or any suitable polymer hinge, so long as it can alter its shape when met by force.

In an alternative embodiment, as shown in FIG. 14, a method of preventing cracking in a pump is disclosed beginning in the Step 300. In the Step 310, a housing is provided having an inlet chamber and an outlet chamber separated by a pumping structure. In the Step 320, a plurality of spaced apart flexible objects are disposed to form at least one wall of the housing such that pressure exerted on the plurality of spaced apart flexible objects increases a volume of the housing. The flexible objects can accommodate a predetermined level of fluid expansion.

The predetermined level of fluid expansion can be between five to twenty-five percent. The flexible objects are preferably spaced apart a predetermined distance. Additionally, the flexible objects are preferably capable of contracting and expanding between a minimum volume condition and a maximum volume condition. The pump can be electro-osmotic. The housing can include rigid plates. Furthermore, the flexible objects can be fastened to the rigid plates. The flexible objects can be made of rubber, plastic or foam.

In another embodiment, shown in FIG. 15, an apparatus or pump 400 includes a housing 410 having hourglass-shaped inlet and outlet chambers. The hourglass-shaped chambers can have a relatively narrowed middle or central portion 405 and substantially identical expanded end portions 407. A pumping structure 420 separates the inlet and outlet chambers from a bottom surface of the housing 410 to an upper surface of the housing 410. The apparatus can include a thermal path from a location of initial freezing to its surroundings.

As the fluid or chamber is cooled from above a freezing point, the thermal path serves to efficiently reject heat stored in the location. For example, an optional metallic insert 430 is mounted from the location of initial freezing in the chamber to the top surface of the chamber would serve. Preferably, the metallic insert 430 is formed of a material that will not contaminate the fluid such as copper. A critical factor is use of any material or structure that assists a particular location become cold fastest, and so that progression of freezing is continuous from that location to the expanded end portions 407 of the chambers. The combination of having hourglass-shaped chambers and the metallic insert 430 allows for freezing to initiate at the narrowed middle or central portion 405 of the hourglass-shaped chambers and expand outward to the expanded end portions 407, where liquid can be further displaced at the inlet, outlet, or both, or a volume accommodating structure can be implemented at the expanded end portions 407 as described above.

In the above-described embodiments, the present invention is applied to a pump or a housing having an inlet chamber and an outlet chamber. Alternatively, the present invention may be applied to any enclosure in a liquid cooling system. The liquid cooling system preferably includes an electro-osmotic pump and a heat exchanger. As such, the size and volume reducing means, the air pockets, the compressible objects, and the compressible objects can be applied to any or each enclosure in the system, including tubing, of the liquid cooling system.

The present invention has been described in terms of specific embodiments incorporating details to facilitate the understanding of the principles of construction and operation of the invention. Such reference herein to specific embodiments and details thereof is not intended to limit the scope of the claims appended hereto. It will be apparent to those skilled in the art that modification s may be made in the embodiment chosen for illustration without departing from the spirit and scope of the invention. 

1. An apparatus for preventing cracking of a liquid system, comprising: a. an enclosure configured to have multiple zones of different freeze susceptibilities and to cause freezing to begin in a high freeze susceptibility zone and for a freeze front to advance from the high freeze susceptibility zone toward a low freeze susceptibility zone through one or more zones of progressively decreasing freeze susceptibility; and b. a compressible object immersed in a zone of lower freeze susceptibility than the high freeze susceptibility zone.
 2. An apparatus for preventing cracking of a liquid system, comprising: a. an enclosure configured to have multiple zones of different surface area to volume ratios and to cause freezing to begin in a high surface area to volume ratio zone and for a freeze front to advance from the high surface area to volume ratio zone toward a low surface area to volume ratio zone; and b. a pressure relief area in the enclosure located in a zone other than the high surface area to volume ratio zone.
 3. The apparatus of claim 2, wherein the pressure relief area is a compressible object.
 4. A freeze-tolerant heat exchanger, comprising: a. a micro-structured heat exchange region having a first freeze susceptibility; b. a manifold region configured to have a second freeze susceptibility so that fluid within the manifold region freezes later than fluid within the micro-structured heat exchange region; and c. a fluid input region including a compressible object and configured to have a third freeze susceptibility so that fluid within the fluid input region freezes later than fluid within the manifold region; wherein the heat exchanger is configured so that a freeze front advances from the micro-structured heat exchange region towards the compressible object.
 5. The freeze-tolerant heat exchanger of claim 4, wherein the micro-structured region comprises one or more of the following: microchannels, microporous foam, and pseudo-foam.
 6. A method of preventing cracking of a liquid system, the system including a pump and a heat exchanger, the method comprising the steps of: a. configuring the system to have multiple zones of different surface area to volume ratios and to cause freezing to begin in a high surface area to volume ratio zone and to advance towards a low surface area to volume ratio zone; b. providing an enclosure fluidly coupled to the system at a zone other than the high surface area to volume ratio zone; and c. placing a compressible object in the enclosure. 